1. Field of the Invention
The present invention relates to an image signal processor that mixes pixel signals generated by pixels covered with color filters arranged in a predetermined array.
2. Description of the Related Art
A multitude of pixels are arranged on a light-receiving surface of an imaging device. Each pixel is covered with a color filter. Certain types of color filters are arranged according to a predetermined array, such as Bayer color array. Each pixel generates a pixel signal according to the amount of a received light component that passes through the color filter. An image signal comprising a plurality of pixel signals is generated when the entire light-receiving surface receives an optical image of an object.
The pixel mixing process is known for the purpose of receiving an object even if an optical image of an object is dark. The pixel mixing process involves signal processing for mixing a plurality of pixel signals generated by pixels that are adjacent to one another. Even if the signal level of a singular pixel signal is low, the signal level of mixed pixel signals is enlarged by the pixel mixing process.
A prior pixel mixing process is explained below using FIGS. 3, 4. Incidentally, in FIG. 3, 4, the rows and columns of the pixel and mixed pixel area are counted from top to bottom and left to right, respectively.
A plurality of pixels are arranged in a matrix on the light-receiving surface. Red, green, and blue pixels 12rp, 12gp, 12bp are covered with red, green, and blue color filters, respectively. Hypothetical zones of red, green, and blue mixed pixel areas 12ar, 12ag, 12ab (2,2) are arranged in a matrix on the light-receiving surface. The red, green, and blue pixels 12pr, 12pg, 12pb are arranged according to the Bayer color array. Further, the red, green, and blue mixed pixel areas 12ar, 12ga, 12ab (2,2) are arranged according to the Bayer color array.
For example, the red mixed pixel area 12ar(1,1) is mapped so that the center of the red mixed pixel area 12ar(1,1) (see FIG. 4) agrees with the blue pixel 12pb(2,2) (see FIG. 3) arranged in the second row from top and second column from left, and its size is four times as large as that of one pixel. Further, the green mixed pixel area 12ag(1,2) is mapped so that the center of the green mixed pixel area 12ag(1,2) agrees with the blue pixel 12pb(2,4) and its size is four times as large as that of one pixel. Further, the blue mixed pixel area 12ab(2,2) is mapped so that the center of the blue mixed pixel area 12ab(2,2) agrees with the blue pixel 12pb(4,4) and its size is four times as large as that of one pixel.
When the pixel mixing process is carried out for the red, green, and blue pixel signals, the same number of red, green, and blue pixel signals are separately summed up.
For example, the red mixed pixel signal of the red mixed pixel area 12ar(1,1) is generated by mixing together four red pixel signals individually generated by the red pixel in the first row and first column 12pr(1,1), the red pixel in the first row and third column 12pr(1,3), the red pixel in the third row and first column 12pr(3,1), and the red pixel in the third row and third column 12pr(3,3).
Similarly, the green mixed pixel signal of the green mixed pixel area 12ag(1,2) is generated by mixing together four green pixel signals individually generated by the green pixel in the first row and fourth column 12pg(1,4), the green pixel in the second row and third column 12pg(2,3), the green pixel in the second row and fifth column 12pg(2,5), and the green pixel in the third row and fourth column 12pg(3,4).
Likewise, the blue mixed pixel signal of the blue mixed pixel area 12ab(2,2) is generated by mixing together four blue pixel signals individually generated by the blue pixel in the second row and fourth column 12pb(2,4), the blue pixel in the fourth row and second column 12pb(4,2), the blue pixel in the fourth row and sixth column 12pb(4, 6), and the blue pixel in the sixth row and fourth column 12pb(6,4).]
According to the above prior pixel mixing process, the distance between the center of the mixed pixel area and the individual red, green, and blue pixels to be mixed together are different from each other. Consequently, the captured image may be blurred.
The distance from the center of the red mixed pixel area to the center s of the individual red pixels whose pixel signals are used for generating the red mixed pixel signal corresponds to 1.41 pixel lengths. The distance from the center of the green mixed pixel area to the centers of the individual green pixels whose pixel signals are used for generating the green mixed pixel signal corresponds to 1 pixel length. The distance from the center of the blue mixed pixel area to the centers of the individual blue pixels whose pixel signals are used for generating the blue mixed pixel signal corresponds to 2 pixel lengths. Because the distance for the blue is especially long, the blue component of the entire captured image will consequently be blurred more than the red and green components.